The term “Tornado Valley” is in reality simply a nickname that has been given by the media to the area of the United States where the incidences of what the expert weather observers describe as significant tornados appears to be most prevalent, in other words where the greatest danger from tornados exist.
The problem with the geographical description of this alley is that its boundaries depend upon the tornado definition that is being used. For example, when the data includes all types and levels of tornados the valley experience will stretch from South-eastern Mississippi to Southern Oklahoma. However, when one relies upon tornado data that is relative to specific types of tornados, being their strengths, this valley takes on the shape of an L shape, with western Iowa being included.
Tornados can of course occur in many areas of the US and indeed other countries as well. However, what has led to the development of the “Tornado Valley” syndrome is the impact the tornado is likely to have upon the population and property within a given area. Hundreds, if not thousands, of tornados do little if any damage in this respect, because their strength is below the level capable of achieving this impact. However, there are others, ranging from strong to violent, that can cost significant loss of life and $ billions of property damage. These stronger tornados have been classified by a scale from F2 (strong) through to F4 (violent).
The geographical definition of “Tornado Valley” has been based upon research, which has tried to determine the likelihood of tornados that fall within the F scales occurring within a given area and the regularity with which this might happen. For example the “Storm Prediction Center” and others, such as Concannon, have used the probability of F2 to F4 scale tornados occurring as their basis for designating the “L” shaped version of Tornado Valley. The term likelihood in this respect relates to the number of days per century and the time of the year, as calculated by the use of historical data. Those areas within Tornado Valley will have a likelihood of F strength tornadoes occurring at least 25 days per century, with the highest areas of risk rising to 40 days and beyond.
In terms of the time of year, an example could be Oklahoma, which suffered a violent tornado event in May 1999. On average, between the end of February and the beginning of July the chance of a tornado affecting this area rises from 0% in late February to 0.06% at the beginning of May before falling back to the 0% position by the beginning of July. Similarly, Oklahoma has a rise in likelihood of tornadoes occurring between mid October and mid December. North-western Alabama has a similar time of year position, although in both cases the percentage of likelihood is smaller, for example their May position is deemed as a 0.04% chance of strong to violent tornados occurring.
Although, as stated in the introduction, tornadoes can occur outside of this area of concentration, even those of a violent nature, the usefulness of defining the areas where they are most likely to occur, such as “Tornado Alley” aids the development of programmes for public safety and assists insurance companies is calculating risk.
Sources:
http://www.spc.noaa.gov/faq/tornado/
http://www.nssl.noaa.gov/users/brooks/public_html/concannon/
